British Open champion Phil Mickelson flirted with a 59, but settled for an 8-under-par 63 and a share of the first-round lead at the second PGA Tour playoff tournament on Saturday (AEST) alongside England's Brian Davis.

Mickelson had eight birdies and an eagle along with two bogeys to set the early pace, and Davis joined him atop the leaderboard with a blemish-free round that featured eight birdies.

They were one stroke in front of American Kevin Stadler, with Spain's Sergio Garcia, and Americans Hunter Mahan and Roberto Castro a further shot back on 65.

Australia's Adam Scott - who started the playoffs with a win at last week's opening tournament - could not stay with the pace as he battled to a 2-over 73.

Scott, who moved to number two in the world with his win last week at Liberty National, went 2-over with bogeys at the 10th and 14th, picked up his only birdie of the day at the par-three 16th, then gave the shot back with a bogey at the par-four sixth.

A group of 16 players on 5-under 66 included recently crowned PGA Champion Jason Dufner, England's Lee Westwood and Ian Poulter, South African Ernie Els, Sweden's Jonas Blixt and 2009 British Open champion Stewart Cink of the United States.

Mickelson was the class of the morning, teeing off on the 10th hole at TPC Boston alongside Scott and world number one Tiger Woods.

He birdied seven of his first nine holes. A bogey from a bunker at the first was followed by an eagle three at the par-five second, where his second shot landed two feet from the pin. He nabbed another birdie at the par-three eighth, but closed with a bogey when he hit his tee shot at the ninth into thick brush on the right.

The tee shot on nine and his wedge at the first were the "only two bad swings" of his day, Mickelson said.

The affable left-hander, who had a 59 in his sights when he carded a first-round 60 at Phoenix this year, was pleased with his round.

"I putted really well," he said. "There were a couple that didn't go that had every bit as good a chance to go as the ones that did and that's the sign that you're putting well."

Davis was delighted with his effort, which he capped by rolling in a 25-foot birdie putt at the ninth.

"I played great today. From tee to green it was good. Got up and down a couple of times and putted really solid today," said Davis, who came into the tournament at 80th in the playoff standings and needing a solid performance to join the top 70 in the standings who advance to next week's playoff event.

Woods, who battled back pain in a runner-up finish to Scott last week, carded a 3-under-par 68 that included four birdies and a bogey.

He said his back was "all good" especially after an extra day off because of the $8 million tournament's Saturday (AEST) start.

The event, the second of four in the US PGA Tour's season-ending playoffs, will conclude on Tuesday (AEST).

"It helped having that extra day off," Woods said. "It made a difference."

Things were even more of a struggle for the third man in the group, as Defending champion Rory McIlroy, trying to turn around a disappointing season, carded a one-under 70 that he called "sloppy".

"I'm making a lot of birdies, but also making a lot of stupid little mistakes," said McIlroy, who is without a win in 2013. "If I can cut those out, it's a different story."